1. Field of the Invention
The present invention relates to a process for producing a grain oriented electrical steel sheet having excellent magnetic properties for use as an iron core for transformers or the like.
2. Description of the Prior Art
A grain oriented electrical steel sheet is used mainly as an iron core material for transformers and other electrical equipment and should be excellent in magnetic properties, such as an excitation property and an iron loss property. The magnetic flux density, B.sub.8, at a magnetic field strength of 800 A/m is usually used as a numerical value for expressing the excitation property. The iron less per kg obtained when the steel sheet is magnetized to 1.7 tesla (T) at a frequency of 50 Hz, i.e., W.sub.17/50, is used as a numerical value for expressing the iron less property. The magnetic flux density is the most dominant factor for the iron loss property. In general, the higher the magnetic flux density, the better the iron loss property. In some cases, an increase in the magnetic flux density causes the size of the secondary recrystallized grain to be increased, so that the iron loss becomes poor. Even in this case, the iron loss property can be improved independently of the grain diameter of the secondary recrystallized grain by using the magnetic domain control.
The grain oriented electrical steel sheet is produced by causing a secondary recrystallization in the final annealing to develop the so-called "Goss texture" having a &lt;001&gt; axis in the rolling direction and a {110} plane on the surface of the steel sheet. In order to obtain good magnetic properties, it is necessary to highly arrange the &lt;001&gt; axis which is an easily magnetizable axis in the same rolling direction.
Representative examples of the process for producing the above-described grain oriented electrical steel sheet having a high magnetic flux density include a process disclosed in Japanese Examined Patent Publication (Kokoku) NO. 40-15644 by Satoru Taguchi et al., and a process disclosed in Japanese Examined Patent Publication (Kokoku) No. 51-13469 by Takuichi Imanaka et al. In the former, MnS and AlN are used mainly as an inhibitor, while in the latter, MnS, MnSe, Sb, etc., are used mainly as the inhibitor. Therefore, in the current technique, it is requisite to properly control the size, form and dispersed state of the precipitate which functions as the inhibitor. With respect to MnS, in the current process, MnS is once completely dissolved in a solid solution form during heating of the slab before hot rolling, and precipitation of MnS is conducted during hot rolling. In order to completely dissolve MnS having an amount necessary for causing the secondary recrystallization, a temperature of about 1400.degree. C. is necessary. This temperature is at least 200.degree. C. above the slab heating temperature of common steels. The slab heating treatment at a high temperature has the following disadvantages.
1) It is necessary to use a high temperature slab heating furnace for exclusive use in the grain oriented electrical steel. PA1 3) The amount of molten scale increases, which has a large adverse effect on the operation, such as the necessity of raking out slag from the slab heating furnace. PA1 (1) a technique which enables the Si content to be increased and, at the same time, a sharp {110}&lt;001&gt; in the secondary recrystallized texture to be ensured by increasing the Al content or increasing the partial pressure of nitrogen in an annealing atmosphere in a temperature region where the secondary recrystallization proceeds; PA1 (2) a technique wherein, in order to more stably attain a proper reduction ratio in the final cold rolling, pre-cold rolling is effected with a proper reduction ratio followed by annealing while avoiding the occurrence of recrystallization as much as possible; and PA1 (3) a technique wherein the surface of the steel sheet is smoothed by using an annealing separator less reactive with Si.sub.02.
2) An energy unit of the slab heating furnace is high.
The above-described problems can be avoided by lowering the slab heating temperature to that used in common steels. This, however, means that MnS effective as the inhibitor is used in a reduced amount or is not used at all, which inevitably renders the secondary recrystallization unstable. For this reason, in order to realize the heating of the slab at a low temperature, it is necessary to strengthen the inhibitor with a precipitate other than MnS for the purpose of sufficiently inhibiting the growth of normal grains during final annealing. Sulfides and further nitrides, oxides, grain boundary segregation elements, etc., are considered effective as the above-described inhibitor, and the following are examples of known techniques associated therewith.
Japanese Examined Patent Publication (Kokoku) No. 54-24685 discloses a method wherein the slab heating at a temperature in the range of from 1050.degree. to 1350.degree. C. is made possible by incorporating, in the steel, a grain boundary segregation element, such as As, Bi, Sn or Sb. Japanese Unexamined Patent Publication (Kokai) No. 52-24116 discloses a method wherein the slab heating at a temperature in the range of from 1100.degree. to 1260.degree. C. is made possible by incorporating, in the steel, a nitride forming element, such as Zr, Ti, B, Nb, Ta, V, Cr or Mo, in addition to Al. Japanese Unexamined Patent Publication (Kokai) No. 57-158322 discloses a method wherein the heating of a slab at a low temperature is made possible by lowering the Mn content so as to have a Mn/S ratio of 2.5 or less and, at the same time, the secondary recrystallization is stabilized by adding Cu. Further, a method wherein the strengthening of the inhibitor is combined with an improvement in the metallic structure has also been disclosed. Specifically, in Japanese Unexamined Patent Publication (Kokai) No. 57-89433, the heating of the slab at a low temperature of 1100.degree. to 1250.degree. C. is made possible by combining the addition of Mn and an additional element, such as S, Se, Sb, Bi, Pb, Sn or B, with the percentage columnar crystal of the slab and the reduction ratio in the second cold rolling of the slab. Further, Japanese Unexamined Patent Publication (Kokai) NO. 59-190324 discloses a method of stabilizing the secondary recrystallization which comprises providing an inhibitor composed mainly of S or Se and Al and B and nitrogen and subjecting the inhibitor to pulse annealing at the time of the primary recrystallization annealing after cold rolling. Thus, a great effort has hitherto been made to enable the slab to be heated at a low temperature in the production of grain oriented electrical steel sheets.
The above-described Japanese Unexamined Patent Publication (Kokai) No. 59-56522 discloses that a slab can be heated at a low temperature when the contents of Mn and S are 0.08 to 0.45% and 0.007% or less, respectively. This method has solved the problem of occurrence of a linear poor secondary recrystallization of products attributable to the coarsening of slab grains during heating of the slab at a high temperature.
However, the method wherein the slab is heated at a low temperature aims primarily at lowering the production cost, and it is a matter of course that commercialization cannot be realized unless the technique enables good magnetic properties to be stably obtained.
An object of the present invention is to provide a technique which enables good magnetic properties to be stably obtained on the condition that the heating of the slab is effected at a low temperature.